This invention relates generally to memory devices and, more particularly, to phase change memory devices and particular bottom electrode configurations thereof.
Phase-change memory (also known as PCM, PRAM, PCRAM, Ovonic Unified Memory and Chalcogenide RAM C-RAM) is a type of non-volatile computer memory. PCM devices use the unique behavior of chalcogenide glass, which can be “switched” between two states, crystalline and amorphous, with the application of heat. PCM memory devices are one of a number of new memory technologies that are attempting to compete in the non-volatile role with the almost universal Flash memory, which has a number of practical problems these replacements hope to address.
Chalcongenide glass is an example of a material that shall be referred to herein as a “phase-change material.” A phase-change material is characterized in that it exists in two main states, crystalline and amorphous. The amorphous state has a high resistance and the crystalline state has a lower resistance by comparison. As such, the material may be used to represent one of two binary states.
To convert the material from the crystalline to the amorphous state (i.e., to “reset” the material) a high current may be applied such that the temperature of the phase-change material exceeds about 600 degrees C. This requires a pulse of current lasting only a few nano-seconds to be applied to the phase change material.
To “set” the phase-change material, i.e., to convert it from its amorphous state to its crystalline state, the phase-change material is heated to a temperature below about 400 degrees C. and above about 200 degrees C. and held at that temperature for some duration of time and then allowed to cool according to the shape of the decay of the pulse applied thereto. In some instances, the total time required to set the phase-change material can be up to 100 nano-seconds or more. In short, at present the set time for a phase-change material is substantially longer than its reset time. It would, therefore, be desirable to reduce the set time for a phase-change memory device.